1. Field of the Invention
This invention relates to apparatus and methods for testing static and kinetic frictional coefficients of sheet material and, more particularly, an automated horizontal plane apparatus for testing static and kinetic frictional coefficients of sheet material and methods related thereto.
2. Description of the Prior Art
It can be appreciated that the determination of frictional characteristics of sheet materials, such as paper, film, rubber, plastics, wood, linoleum, and coatings, is crucial to evaluate the workability of the sheet materials in processes and by the end consumer. For example, paper photocopying machines are formed with rollers and paper feeding mechanisms which are designed to cooperatively feed sheets of paper having frictional coefficients within certain predetermined ranges. If the frictional coefficients are outside of these ranges, the photocopying machine may fail to feed the paper sheets or may feed the paper sheets irregularly. As a second example, linoleum sheets used as flooring must be formed with certain frictional characteristics which will allow a person to comfortably and safely walk thereupon.
Horizontal plane testing apparatus have been developed in the prior art to test static and kinetic frictional coefficients of sheet materials. The prior art devices include a horizontal platen with a clamping mechanism for rigidly fixing a sample of the sheet material to the platen. Typically, frictional coefficient testing apparatus are used to test the frictional coefficients between two samples of the same sheet material, although the frictional coefficients between two different sheet materials may also be tested. A second sample of sheet material is secured to a planar face of a sled. The sled is a generally parallelepiped shaped block having a predetermined weight. In the prior art, the sled is manually placed on the platen, with the two samples of sheet material being in face-to-face engagement, at one end of the first sample of sheet material. The sled is driven across at least a portion of the length of the first sample of sheet material at a constant speed along a single linear axis, and a load cell is coupled to the sled to measure the force required at all times to drive the sled across the length of the sheet material. A data acquisition system receives data measured by the load cell and calculates static and kinetic coefficients of friction for the interface of the two samples of sheet material. The static coefficient is an indicator of the degree of force required to initially move or "slip" the sled from a stationary position in a single coordinate direction, and, thus, it is calculated during the initial portion of the test, with the sled resting motionless on the platen. The value of the static coefficient is obtained by dividing the amount of force required to initially move the sled by the weight of the sled. In contrast, the kinetic coefficient is an indicator of the degree of force required to continue movement of the sled in a single coordinate direction, with the sled already being in motion. During the test, the kinetic coefficient may be determined once the sled is in motion and is calculated by dividing the amount of force required to continue movement of the sled by the weight of the sled.
An example of a prior art horizontal plane static and kinetic friction coefficient testing apparatus is manufactured and sold under the tradename "MONITOR/SLIP & FRICTION.TM." by Testing Machines, Inc. of Islandia, N.Y.
The prior art apparatus suffers from several drawbacks. First, the manual placement of the sled onto the platen results in testing errors and lack of testing repeatability due to varying forces being applied to the interface of the tested samples of sheet materials by an operator or operators of the apparatus. A slight difference in the degree of force applied in placing the sled onto the platen, greater or smaller, may translate to respectively greater or less meshing of the engaged surfaces of the samples, with corresponding increases or decreases in the measured values of the static frictional characteristics of the samples. Consequently, errors are created in measured data. Furthermore, even with an apparatus accurately measuring coefficients, the magnitude of the frictional coefficients will vary from test to test of the same samples due to the differences in pressures applied in placing the sled for each test. As such, the determination of the actual coefficients of a sheet material is quite difficult, requiring repeated testing of the same sheet material and intense scrutiny of such test results. Thus, there is a need for a testing apparatus which can measure static and kinetic frictional coefficients of a sheet material with repeated accuracy. It should be noted that absolute test repeatability is not required where exact test results are duplicated; however, test results should repeat within a statistically acceptable range. As used herein, "repeatable" and "repeatability", respectively, refer to test results obtained, and the ability to obtain test results, respectively, from the same samples which fall within statistically acceptable ranges of each other.
Second, manual placement of the sled onto the platen may result in misalignment of the sled relative to the platen. Since the force used to determine the static and kinetic frictional coefficients is linearly applied to the sled along a single axis, to obtain repeatable test measurements of a static frictional coefficient, the sled must be aligned in the same manner relative to the linear axis of the force for each test. Manual placement of the sled often results in misalignment, with attendant distorted results and lack of test repeatability. Also, the linear force is typically applied through a single point coupling with the sled. If the sled is not fully coaxially aligned with the axis of the linear force, the coupling acts as a fulcrum about which the sled rotates slightly into alignment with the linear axis during initial movement. The rotational motion of the sled relative to the platen generates resistive frictional forces in two coordinate directions, one coordinate direction being parallel to the linear axis and the second coordinate axis being perpendicular to the linear axis. Since the coefficients of friction are determined relative to the degree of force required to move the sled in one coordinate direction, the additional frictional forces generated in the second coordinate direction distort the test results. It can be appreciated that manual placement of the sled onto the platen often results in misalignment of the sled relative to the platen and subsequent slight rotational movement of the sled, resulting in a lack of repeatability in tests and inaccurate frictional coefficient measurements.
Third, a mechanism to control the dwell time of the sled on the platen relative to the actual start of the test is not present in the prior art apparatus. Once the sled is manually placed on the platen, the prior art apparatus must be manually actuated. However, the dwell time, the amount of time the sled rests on the platen prior to initiation of a test, affects the outcome of the test. With certain materials, "blocking", which is additional resistance to movement beyond friction, may occur as a result of molecular adherence between the tested materials caused by excessive dwell time. During such tests, forces applied to the sled must overcome not only friction, but also molecular adherence, to move the sled and, consequently, test results will be distorted. If there is an inadequate amount of dwell time, air trapped between the two samples of sheet material may not have an opportunity to evacuate, and a layer of air may be entrapped between the two samples during testing. The layer of air will act as a cushion and lessen the frictional forces generated between the two samples. Attempts have been made in the prior art to manually monitor dwell time to achieve repeatability of test results. As with the manual placement of the sled onto the platen, manual measurement of the dwell time is affected by human error, resulting in a failure to achieve testing repeatability.
Fourth, prior art horizontal plane frictional coefficient testing apparatus are configured to drive the sled during the course of the test at a single speed from initiation to completion. Each test run includes two portions, an initial static coefficient test portion and a subsequent kinetic coefficient test portion. The simultaneous end of the first portion of the test and the beginning of the second portion of the test is an instantaneous point in time at which the sled initially moves relative to the platen. The use of a constant test speed is appropriate for determining the kinetic coefficient of friction, however, the constant rate may lead to inaccuracies in determining the static coefficient of friction. With the application of the constant rate of speed from initiation of the test, a jolt or shock is applied to the sled initially which may distort force readings collected during the static coefficient phase of the test. Also, with a constant speed test, the force is applied to the sled with a constant rate. Since the capture of data relating to the instantaneous point of initial movement of the sled is essential to determining the static coefficient of friction, a slow speed, and thus a slow rate of applying force, is preferred to ensure accurate measurement of past data. However, with the amount of time to perform a test also being a factor, apparatuses are typically configured to run at a constant speed which generates a rate of force application greater than the aforementioned preferred rate. Repeated test runs at a constant rate slow enough to ensure accurate static frictional coefficient measurements may be overall too time-consuming in a commercial environment.
Fifth, prior art apparatus utilize rigid coupling systems which are not adaptable to the two portions of the testing procedure. A rigid coupling is preferred for the kinetic coefficient testing portion which allows constant transmission of the driving force to the sled, whereas, in contrast, some elasticity is preferred in the coupling for the static coefficient portion of the test. An elastic coupling would allow a load to be applied to the sled gradually, rather than abruptly.
A sixth drawback in the prior art is the rearward translation of the sled relative to the platen due to manual placement and/or removal of the sled from the platen. As described above, prior to a test run in the prior art, the sled is manually placed onto the platen. However, often the sled is manually placed on the platen with subsequent movement of the sled in an opposite direction from the direction in which the sled is to be translated during the test run. As a result, the friction between the interengaging faces of the samples of sheet material causes the grain of the surfaces of the samples to be urged into the opposite direction. Since several tests are often performed on the same samples of sheet material, the disturbed grain of the sample surfaces undermines repeatability in test results. Furthermore, manual removal of the sled from the platen after a test run may also cause rearward movement of the sled relative to the platen and further disruption of the grain of the sample surfaces. To enhance test repeatability, "back sliding" of the sled relative to the platen should be avoided.
It is an object of the subject invention to provide a horizontal plane frictional coefficient testing apparatus which overcomes the shortcomings of the prior art devices and measures static and kinetic frictional coefficients with repeatability.
Also, it is an object of the subject invention to provide a horizontal plane frictional coefficient testing apparatus which provides for automated reversible lifting and lowering of a sled.
It is also an object of the subject invention to provide a frictional coefficient testing apparatus which ensures proper alignment of the sled relative to the platen of the apparatus.
It is a further object of the subject invention to provide a horizontal plane frictional coefficient testing apparatus which operates at more than one speed during a single test and is provided with a semi-rigid coupling for the sled which reacts to the operating speed of the apparatus.
Yet another object of the subject invention is to provide an apparatus which monitors dwell time of the sled on the platen and initiates test runs at the lapse of predetermined dwell times.
It is a further object of the subject invention to provide a horizontal plane frictional coefficient testing apparatus which prevents translation of the sled before and after a test run in a direction opposite the testing direction.